Technical documents

An overview of minimum flows and levels

One way the St. Johns River Water Management District is working to protect Florida’s water resources is through the minimum flows and levels (MFLs) program. Establishing MFLs is an important goal in the District’s work of planning for adequate water supplies for today and for future generations while also protecting the District’s water resources. The District is setting MFLs for lakes, streams, rivers, wetlands, springs and aquifers.

What are MFLs?

MFLs are the minimum water flows and/or levels adopted by the District Governing Board as necessary to prevent significant harm to the water resources or ecology of an area resulting from permitted water withdrawals. MFLs define how often and for how long high, intermediate and low water flows and/or levels should occur to prevent significant harm. Two to five MFLs are typically defined for each system. The names of the MFLs are minimum infrequent high, minimum frequent high, minimum average, minimum frequent low and minimum infrequent low (see Figure 1).

Figure 1

Figure 2 represents two hydrographs depicting the fluctuation of high and low water levels or flow in a typical stream or lake over a long time period. The upper line represents the existing hydrologic conditions and the lower line represents the hydrologic conditions defined by the MFLs. The hydrologic conditions defined by the MFLs are similar to, but are usually lower than, the existing hydrologic conditions.

Figure 2

These two hydrographs can be summarized as the percentage of time each water level or flow is equaled or exceeded; this is called a water level or flow duration curve (Figure 3). The area below the MFLs curve (salmon-colored shaded area in Figure 3) represents the water reserved for protection of fish and wildlife or public health and safety. When use of water resources shifts the water levels below that defined by the MFLs, significant ecological harm is expected to occur.

Figure 3

The distance between the two curves (light blue shaded area in Figure 3) represents the water available for use that will not result in harm to the water resources.